Clear it in 3 steps
Start the environment
Investigate the target
Submit the flag

An SSL Strip attack downgraded HTTPS to HTTP, so the victim's credentials traveled in clear text and were captured. Download the packet capture (traffic.pcap), analyze the clear-text HTTP POSTs, and extract the login password (the flag).
SSL Strip downgraded HTTPS to HTTP, so credentials traveled in clear text
Inspect each POST body (http.request.method == POST); newsletter/search are decoys
The password field in the POST /login body is the flag